In general, the term “fouling” or “biofouling” (biological overgrowth and aufwuchs) is used to refer to the undesirable accumulation or solid materials (marine organisms: bacteria, algae, shellfish, barnacles etc.) on rigid boundary surfaces. Antifouling measures help to prevent fouling on structures that are surrounded by marine or salt-containing water, or liquid media containing salt (“seawater”), or are at least wetted with such water intermittently or constantly. Offshore structures are usually built from steel or concrete, and are nearly always covered completely in a layer of fouling, particularly in the intertidal area. As a result, the area that is exposed to wave energy is enlarged, the surface of such structures is permanently covered so that it may be attacked or corroded, and the biological mass is itself increased locally by the aufwuchs. Inspection activities are hindered. Moreover, aufwuchs that falls off can cause oxygen depletion on the seabed, particularly in areas with little or no current movement, and negatively impact marine animal communities. Antifouling measures also help to protect wooden bodies in the water, such as mooring posts in marina parts, from clinging and boring organisms. Wooden elements can be colonised by various organisms, which can completely cover the elements and thus impair their function. In general, fouling with clinging or adhering organisms can thus destroy the surface of a structure, and this has led to more intensive measures for combating fouling, known as antifouling. Besides mechanical cleaning methods and special antifouling paints or coatings, electrochemical antifouling systems have also been developed, and a major advantage of these is that they are non-toxic.
Electrochemical antifouling systems are based on electrolysis in seawater. A direct current flowing between the anode and the cathode causes the formation of products of dissociation (cathode H+, anode OH−), which in turn raise the local pH values at the boundary surface between the electrode and the seawater (cathode basic, anode acidic). A suitable current regime for causing electrolysis at the cathode to prevent microbiological or calcareous fouling on conductive or semiconductive surfaces that are exposed to seawater is described in detail in U.S. Pat. No. 4,440,611.
DE 41 09 198 C2 describes applying a coating consisting of a binder and macromolecules with free anionic or cationic groups in the molecule to the surface that is to be protected. By controlling the DC voltage, the products of dissociation can be caused to accumulate out of the seawater and set up a specific pH value on this surface. By applying a voltage of 0.3 V/cm2, the pH value can be raised to basic values of pH 9-10 due to protonation at the cathode. For purposes of antifouling protection, it is known from DE 41 09 197 C2 to switch the polarity of the DC voltage continuously according to a random principle, causing the pH values to change between acidic and basic. This also repels organisms that are able to tolerate constantly strong basic or acidic pH values.
A similar antifouling system is described in DE 698 02 979 T2, in which a layer having a streaked structure or thin metal strips are applied underneath a continuous conductive layer having a different resistance behaviour, so that the current density created can be adjusted specifically depending on the nature of the overgrowth. An antifouling system including electrode plates positioned in front of the surface to be protected is described in JP 2004-278161 A. It is also described in JP 2004-270164 A to secure these electrode plates by supporting them in rails.
Another consequence of electrolysis in seawater is that minerals are also precipitated on the cathode (mineral accretion). This is described in U.S. Pat. No. 5,543,034. This precipitate is particularly hard aragonite (a polymorph of calcite, calcium carbonate CaCO3, mohs hardness from 3.5 to 4.5 and noticeable cleavage in one direction) and soft brucite (magnesium hydroxide Mg(OH)2, mohs hardness 2 to 2.5, easily cleaved in one direction). The deposit of hard aragonite in particular can be used to build artificial reefs (biorock technology), on which the growth of aquatic organisms is deliberately encouraged. The rise in pH value of 0.1 compared with equilibrium (average pH value 8.2) that is observed when the aragonite is deposited also promotes increased growth of the organisms that are to be encouraged. The cathode is also protected from corrosion by lime deposits.
Based on the biorock technology, in order to create an artificial reef, it is described in the EAT reports (2001 NOMATEC Project, subject “Electrochemical Accretion Technology” (EAT), Introduction and progress reports, 1st and 2nd project years, version of 29.12.2004, available on the internet at URL http://www.uni-due.de/nomatec/index_de.html, version of 29.09.2009) to use a framework of steel, preferably a thin wire mesh, as the cathodic matrix for electrolytic lime precipitation. A titanium lattice is used as the anode. It will be observed that the accretion of relatively soft brucite, which hinders reef formation, is an indication of high current densities in the cathode. The deposit of brucite can be counteracted by using larger cathodic surfaces.
In addition, DE 10 2004 039 593 B4 describes a method for extracting brucite from seawater by electrolysis, in which the accretion of brucite is supported deliberately by adding a magnesium salt solution. In order to promote the accretion of brucite, the current density at the cathode must be adjusted such that a pH value of at least 9.7 (with normal seawater) is achieved. In this context, it was observed that the accretion product brucite in its crystalline form is obtained with a relatively low current density at the cathode, and if a higher current density is used, brucite is precipitated in its soft, soap-like form.
JP 07-268252 describes a species-related antifouling system with a limp net through which a current is passed, serving as a water inlet channel. The net is stretched across the channel and along the channel walls in the manner of a trap. This forms the lattice electrode, and is connected as the anode. The cathode has the form of a rod and is arranged at a distance from the anode in the seawater. The cathode is not susceptible to decomposition and is therefore made from a material that is not corrosion-resistant, for example iron. However, in order to prevent the anode from being stripped away during electrolysis, the net cable is of special construction. It consists of three strands, each of which is constructed from non-conductive monofilaments surrounding a conductive metal foil of titanium or a titanium-aluminium plated material as its core. All three strands are embedded in a plastic sheath that is rendered electrically conductive by the addition of platinum powder or titanium powder. To protect the channel wall from fouling with an anodic protection, the channel wall must be electrically conductive. The current is supplied to the net, in particular to the metal foil in the core of the strands, via the electrically conductive channel wall. The net can only be removed from water inlet channel by dismantling it manually, which means that personnel must be available at the location.